Biofilm formation is important in the pathogenesis of acute and chronic infections of the lung by Pseudomonas aeruginosa, decreasing the effectiveness of host defenses and bacterial susceptibility to killing by antibiotics. Recent evidence shows that limiting iron (Fe) availability prevents P. aeruginosa biofilm formation and enhances its susceptibility to tobramycin. Gallium (Ca), a group IliA transition metal disrupts the Fe metabolism of many cell types. We have shown that Ga inhibits the growth and Fe acquisition by Mycobacterium tuberculosis. Based on these and new data, we hypothesize that Ga can disrupt P. aeruginosa Fe acquisition/metabolism, thereby disrupting biofilm formation and enhancing susceptibility to conventional antibiotics. It may also limit the growth of planktonic bacteria that would be in dynamic equilibrium with the biofilm at sites of P. aeruginosa infection. Thus, Ga could become a potent therapy against P. aeruginosa biofilms. To test these hypotheses, two specific aims will be pursued. First, we will confirm and define the mechanism whereby Ga inhibits P. aeruginosa biofilm formation and ascertain if Ga enhances the susceptibility of P. aeruginosa to conventional antibiotics. This will be done using two in vitro P. aeruginosa biofilm models. Second, we will determine the effect of Ga on P. aeruginosa Fe acquisition and Fe-dependent gene regulation. We will directly measure Fe and Ga acquisition by P. aeruginosa, as well as characterize the interaction of Ga with its siderophores. DNA microarray technology will be used to study the impact of Ga on P. aeruginosa gene regulation. The long-term goal of this work is to develop a means to utilize Ga for the prevention and/or treatment of P. aeruginosa biofilms and the infections that result. We would envision the systemic and/or aerosolized pulmonary delivery of Ga to P. aeruginosa-infected patients and/or impregnating endotracheal tubes with Ga to prevent biofilm formation in ventilated patients.